Question

Lactic acid is very good at penetrating the skin. Because of this, it is used in a number of products for skin care, such as exfoliating creams and chemical peels, and in formulations for transdermal delivery of medications, such as topical ointments and extended release patches for delivery through the skin. Lactic acid has a pKa of 3.86 in water at 25o C.

(a) The initial formulation being tested is a 1.0 wt% solution of Poly-XSeption-L® in a 0.015 M solution of aqueous lactic acid. Neglecting the effect of the antibiotic on the solution, what would you estimate the pH of this solution to be? State any assumptions you make.

(b) In clinical studies, it was found that the acidity of the initial formulation caused some problems with local irritation, and that Poly-XSeption-L® was not as effective below a pH of 4.2. It is proposed that sodium lactate be added to create a buffered solution with pH = 4.2. What concentration of sodium lactate should be used to achieve this pH?

Answer 1